Anodic oxidation of aluminum and of its alloys



P 12, 1967 D. RODRIGUEZ MARTINEZ 3,341,435

ANODIC OXIDATION OF ALUMINUM AND OF ITS ALLOYS Filed April 30, 1964 FIG. 1 1

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w ww United States Patent 3,341,435 AN ODIC OXIDATION OF ALUMINUM AND OF ITS ALLOYS Dionisio Rodriguez Martinez, Neopatria 84, Barcelona 16, Spain Filed Apr. 30, 1964, Ser. No. 363,782 Claims priority, application Spain, May 4, 1963, 287,693; Feb. 1, 1964, 295,984 7 Claims. (Cl. 204-58) The present invention relates to a process for the surface protection of aluminum and of its alloys by anodic oxidation in a bath of chromic acid.

The surface protection of aluminum and of its alloys by anodic oxidation in a bath of chromic acid has been developed in 1923 by Bengough and Stuart. These authors have used low concentrations of chromic acid, 2.5 to 3.5% by weight, and have obtained thin coatings (generally of 2 to 6 microns) with a good resistance to corrosion. The process of these authors has been a great success but it necessitates the use of a relatively complex circuit in which voltage is a function of time during the course of each operation.

Thereafter the anodic protection of aluminum and of its alloys in a sulphuric acid bath was initiated (Gaver and OBrien). The protective layer of alumina is thicker than in the chromic acid process (in general from 10 to 20 microns); it is less flexible and for equal thickness it is distinctly less resistant to corrosion.

Another process of anodic oxidation in a bath of chromic acid has been developed in the year 1937 by R. W. Buzzard, of the Bureau of Standards of the United States of America. This process uses in practice a chromic acid with a concentration comprised between and by weight and a temperature of 30 to 40 C., and allows working under constant voltage. This simplification in working explains why it has supplanted the Bengough-Stuart process.

Since then, processes of anodization in a bath of chromic acid under a constant voltage have been the subject of a large number of publications.

A review of these 'works has been published in the German Journal Aluminium, 39. Iahrg. March 1963, pages 169 to 180, by F. Modic.

One knows that processes using chromic acid under constant voltage have, by comparison to the sulphuric acid processes, as a principal disadvantage the cost of chromic acid which is very much greater than that of sulphuric acid. To diminish the specific consumption of chromic acid (that is to say the consumption of chromic acid per unit of anodized surface) has been one of the great preoccupations of research Workers, who have varied up to their extreme limits all those factors which might influence the operation. One knows that the most important cause of deterioration of the bath is the progressive reduction of its chromic acid content, the chrominim ions passing from the hexavalent state to the trivalent state. One has noted that this reduction becomes difficult when one choses a cathodic surface which is as small as possible (loc. cit., p. 179, 1st column), and that, consequently, one operates with a high cathodic current density.

One has stated that the most favorable concentrations in chromic acid were between 10 and Higher concentrations did not allow improvements in the thickness of the coating obtained per unit time, but have, on the contrary, the disadvantage of leading to greater losses by entrainment, and to a more important reduction of the hexavalent chromium to trivalent chromium, relative to the square decimeter of anodized surface (loc. cit., p. 171, 2nd column, paragraph 3.1).

One has shown that when the electrolysis volt-age is increased, the resistance to corrosion of the coating obtained increases equal-1y, and may reach a Very high value (loc. cit. p. 174, 1st column, paragraph 4).

One has equally maintained that treatment temperatures below 40 C. lead to thin coatings, transparent to semi-opaque, which are very difiicult to color and mechanically too fragile (loc. cit. page 1171, 2nd column, paragraph 3.2), and that this condition cannot be sub stantially improved by an extension of the period of operation, even beyond one hour (loc. cit., p. 172, 1st column, paragraph 3.2). Up to 35 C. or below, one obtains wholly colorless layers, transparent or only slightly diffuse (loc. cit., page 172, 2nd column). One needs a bath temperature of at least C. to obtain wholly opaque oxidized layers, and better still, a temperature of to C. (loc. cit., page 173, 1st column, paragraph 3.21). At a temperature below 40 C., the layers obtained have mechanical properties which are devoid of any practical interest (loc. cit., page 173, 1st column, paragraph 3.22).

Subject to finding means for supressing the consumption of the bath (loc. cit., page 178, 2nd column, paragraph 7), one has generally compensated for the destruction of hexavalent chromium by additions of chromic acid in a calculated quantity. However, when at the end of a certain time, the bath is too highly charged in t-rivalent chromium and in aluminum salts, it becomes necessary to replace this bath partly or wholly. One has proposed the purging of the bath by passage over cationexchange resins which fix the dissolved aluminum salts; this procedure does not affect the main factor causing consumption of the bath, namely, the enrichment in salts of trivalent chromium.

One has also proposed the recovery of the chromic acid from the deteriorated baths before these rejected with the used waters. But this mode of operation is complex and costly in practice, so that the majority of users simply throw away the deteriorated baths with the used waters. However, in this case, the chromium of these used waters must be fixed before they are thrown into the river, since public authorities are often extereme- 'ly strict regarding the maximum contents in chromic acid tolerated in the used waters (see La Metallurgie et la Construction Mecanique, volume 95, December 1963, pages 1071 and 1075-1076).

It is known that the objects to be anodized are to be carefully connected up with the anodic cur-rent supply, which brings about important labor costs.

It has been tried to diminish them by carrying out the anodization in barrel or basket, wherein the fixing (attachment) of the objects is more primitive. But this means is only possible for small objects, and gives less regular (less uniform) results than more careful connections.

The present invention has for its principal object to suppress entirely the deterioration of chromic acid baths caused by the reduction of hexavalent chromium to trivalent chromium.

It has for its object to reduce very substantially the specific consumption in chromic acid as Well as the specific consumption of electric energy, and to increase the surface which may be anodized daily in a given bath volume, and at the same time to confer upon the coating obtained qualities which are at least equal to those obtained with the known processes of anodization in a chromic bath.

The process which forms the object of the present invention relates to the protection of the surface of aluminum and of its alloys by oxidation in a chromic acid bath under constant voltage with direct current, the cathodic surface being chosen to be as small as possible,

and is characterized by the following means used in combination:

The concentration in chromic acid is substantially between 15 and 19% by weight;

The temperature of the bath is below 30 C.;

The electrolysis voltage is below 25 volts, and preferably between substantially 21 and 23 volts;

The anodic surface is between substantially 1.7 B dm. and 2.3 B dm. B representing the volume of the bath expressed in liters;

The cathodic surface is below 0.6 B dm. and is preferably between substantially 0.03 B drn. and 0.05 B dm.

The electrolysis is preferably carried out without utilizing any agitation means;

whereby the ratio between the number of atom grams of chromium dissolved in the bath in the hexavalent state to the number of atom grams of chromium dissolve-d in the bath in the trivalent state remains constant, at a value which is always greater than 70 and may exceed 120.

The definition of the invention is in definite opposition to the state of the art given above. In accordance with the prior art, the high concentration in chromic acid applied according to the invention, namely 15 to 19% by weight, is discouraged and in particular the temperature below 30 is described as being definitely to be avoided.

The applicant should therefore have been turned away from its searches by the knowledge of the state of the art in the field of the invention.

The means defined above as constituting the invention can only act in combination. If one withdrew from this combination any one of these means which compose it, the results of the invention, as they will hereinafter be described, would cease completely to be accessible.

To indicate that the features constituting the invention are closely linked one to the other, it should be noted that the density of the anodic current must be considered to be a consequence of the bath temperature (Aluminum, loc. cit., page 173, 2nd column, and page 174, 1st column, paragraph 3.3) as well as of the concentration. This density of anodic current is dependent upon the voltage (according to Joules law), upon the total intensity traversing the electrolysis cell and upon the anodic surface. This total intensity itself influences the density of the cathodic current. It is necessary to take account of this last density when one chooses the cathodic surface.

In addition, the applicant has established by experiments (see below) that reactions of a peroxidic nature take place at the anode under the working conditions which are peculiar to the invention. It is obvious that such reactions are under the influence of the temperature and of the concentration.

The interlocking nature of the features of the invention is most particularly shown by the technical results obtained using the new combination, which are very distinctly separated from that which one could predict according to the prior art, as will be seen later on.

The experiments mentioned above are as follows: if, at the moment when the coating is in the course of becoming opaque, that is to say after 15 to 30 minutes of treatment, one withdraws from the bath a plate of alurninum to be anodized, one notes after rinsing, that the aluminum layer formed is colored in blue; this coloring does not appear at the final stage of treatment after the coating has become entirely opaque. This phenomenon cannot be attributed to an addition of hydrogen peroxide since the baths concerned in these operations have never been subjected to such treatment. These experiments show that peroxidic compounds of chromium appear during the anodic oxidation of the aluminum in a chromic medium (blue peroxidic compounds of hexavalent chromium are mentioned in Gmelins Handbuch der Anorganischen Chemie Syst. No. 52, 8th edition, 1962, part B, pages 152, 153, 156, and Pascal Noveau Trait de 4- Chimie Minrale, volume 14, 1959 edition, pages 77 and Although the invention is not concerned with these observations of purely scientific character, the applicant has nevertheless thought it necessary to report them since, to his knowledge, the observed phenomena were not previously published.

According to the invention, coatings with a thickness of 3 microns are obtained after 60 minutes of treatment; thicknesses of 4.5 microns after minutes; and of 6 microns after minutes. In practice, one rarely exceeds 90 minutes of treatment.

The periods practiced according to the invention are greater than those in use in the conventional processes of chromic anodization under constant voltage, which are generally of 40 minutes for a thickness of 3 microns instead of 60 minutes according to the new process.

But this disadvantage of an increase in the period of operation is largely compensated for by the much greater increase in the anodized surface at each operation, related to 1 liter of bath, the value of this surface being specified in the definition of the invention given above.

According to the applicants experience, this anodized surface is very much greater than those generally employed in industrial practice in the known processes of anodization in a chromic bath under constant voltage.

The coatings obtained according to the invention have a total opacity, which constitutes an unexpected result in complete contradiction to the prior art mentioned above regarding work at a temperature below 40.

The resistance to corrosion of coating obtained according to the new process is superior to that of the coatings obtained according to the known processes of anodization in a chromic acid bath. Based upon the state of the art already mentioned, one would have expected a resistance to corrosion which would have been inferior to that commonly obtained by the processes of anodization in a chromic bath, in view of the relatively low voltage used according to the invention (comparative corrosion tests have been made using the standardized brine mist test, the acetic brine mist test with copper chloride, also known as the Cass test, in addition in an industrial atmosphere, a sea water atmosphere and a sea-water industrial atmosphere).

Tests on resistance to abrasion, sealing tests, tests of bending capacity, and tests of coloring capacity give results which are highly comparable with those obtained according to the conventional processes of anodization in a chromic bath under a constant voltage. The coating obtained according to the invention has an attractive appearance resembling enamel, at least as satisfactory as that obtained according to the previous processes with a chromic bath.

The surfaces treated according to the new process receive a coating which is very uniform and regular, even at points of the anode which are at a very considerable distance from the cathodic surface.

, The nature of the alloys which may be anodized according to the new process is practically the same as that of alloys which may be treated by the known processes of anodic oxidation in a chromic bath under constant voltage.

The fact that the electrolysis is carried out without utilizing any agitation means is conducive to the reduction of the dissolving rate of aluminum, which is much less than in the earlier processes. This dissolving rate, though it is a very little one, is however disturbing in the long run, owing to the accumulating of dissolved aluminum salts in the bath. Therefore the purging of the chromic bath by circulation over cation-exchange resins may be advantageously applied to the invention, so as to fix the dissolved aluminum salts. Although such a purging has been known previously, its combination with the process of the invention is new since it yields a new technicaleffect which will clearly result from the comparison between Examples 1 and 4 hereinafter set forth. This purging may be effected in a continuous or in a discontinuous manner.

One knows that, in the previous processes of anodic oxidation under a constant voltage the consumption of 180 to 200 grams of chromic acid per m of anodized surface was still acceptable (see Aluminum, loc. cit., page 180, first column, paragraph 9). Now, in the new process, the specific consumption of chromic acid is generally below 24 grams per m? of anodized surface, more particularly between 16 and 23 grams per m Such a result constitutes a new and surprising technical effect obtained as a result of the combination peculiar to the invention.

proved by the purging of the bath with ion-exchange resin material as described above.

trivalent chromium nor in salts of aluminum as will be shown by the analyses.

trical energy (evaluated with direct current) which are very low, not exceeding face, for a layer thickness of 4.5 microns, and not exceeding 0.5 kWh/m. for a thickness of 3 microns (which is to be taken into consideration in practice when comparison is made with the previous processes of chromic anodization under constant voltage).

lower than those obtained with the known processes of The figures relates to the process of the invention im- In this process, the bath is enriched neither in salts of The lnvention allows specific consumptions of elec- 0.8 kWh. per m. of anodized sur- Such a consumption of electrical energy is distinctly anodization in a chromic bath.

One may, according to an optional feature of the invention, add to the bath during its first putting into operation a quantity of hydrogen peroxide, calculated as 30 ity: one lays down the object to be anodized on an objccts-holder electrically connected with the anodic current supply. The weight of the object is sufiicient for ensuring a satisfactory electrical contact. Any fixing or strongly applying the said objects against the anode of the electrolytic cell is no longer necessary. Such an improvement, which is equally new against the already cited prior art, has a far-reaching economical effect owing to the important cost decrease involved. Large-sized pieces (objects), namely having the maximal size compatible with the size of the cell, may be treated. FIGURES 1 and 2 are given by way of illustration, but not by way of limitation of the feature of the invention.

FIGURE 1 shows the anodization cell and the objectsholder in vertical and transverse section, FIGURE 2 shows the same in longitudinal section.

In the FIGURE 1, 1 is the cell made of polyvinylchloride, 2 is the bath, 3 the objects-holder connected with the anodic current supply, 4 one of the horizontal branches of the objects-holder, 5 a cross-bar joining the parts of the objects-holder, 6 are objects to be anodized, such as channel bars. It should be understood that other embodiments may be realized without departing from the field of that feature of the invention comprising electrically connecting the objects to be anodized solely by their own weight.

Applications of the new process are recommended in buildings where the attractive appearance of the treated surfaces and their particular resistance to corrosion are of value.

The following four comparative examples will illustrate the invention without however limiting it.

Thefirst three of these examples are schematically set forth by the table below:

Example 1 Example 2 Example 3 New process Prior art; Prior art Thickness of the coating 4.5 microns 4.5 microns 15 microns. Bath composition 200 g. /1. Cr); 80 g./l. CrOa, 200 g./1. H2804.

- 8 g./l. C 0 Hz, I 4 g./l. BO3H3. Surface treated per operation and per 2 dm. 0.22 dm. 0.22 dmfl.

liter of bath. Number of operations per day of 9 hours. 6 13. Surface treated per day and per liter of 12 dm. 2 86 drn. 2.86 dmfl. Lie of the bath expressed in working 500 days 18 days 175 days.

ays. Life of the bath expressed in number of 3,000 235 2,300.

operations. Surface treated corresponding to the exmfl/l. 0.5 mi /l 5 mill.

haustion of 1 liter of bath.

Consumption of bath in grams, owing to the renewal of the bath 160 g. Grog/mi,

3.4 g. CrO /m.

8 g. BO3H3/m. Consumption of the bath, owing to the entrainment loss' In l./rn. 0.09 l./m. 0.091.]m. 0.075 l./m. In g./m. 18 g. CrOs/m. 7 g. (Hos/111. 15 g. H SO4/rn. 0.7 g. CzO4Hz/I1L I 0.4 g. BO3H3/m. Total consumption of the bath in g./m. Comprised between 18 167 g. CrOg/mfl, 55 g. H SO4/m3.

and 21.4 g. CrOz/mfi. 16.7 g. C2O4Hz/m. 8.4 g. BO Halmfi. Consumption in direct current per 111. 0.8 kwh./m. 1.4 kwh./m. 1.56 kWh/131.

treated surface.

volumes hydrogen peroxide, equal to 0.02 to 0.2% by weight or" the bath. As a result of such an addition, the above related equilibrium relationship between the hexavalent chromium and the trivalent chromium is achieved from the beginning of the first operation. If one did not have recourse to such an addition, the advantageous results of the invention could however be reached: it should only be necessary to extend the duration of the first opera: tion, for obtaining coatings as satisfying as those obtained in the following operations.

According to a preferred feature of the invention, very uniform and regular coatings are obtainable when the electrical connection of each object to be anodized with the anodic current supply is carried out merely by grav- This table requires the following complementary explanations;

Example 1 relates to the process according to the invention. The bath contains 200 grams per liter of chromic acid, namely 17.5% by weight; the anodization is effected with 22 volts at 26 C., each operation lasting minutes. The installation is provided with a device for the purging of aluminum salts by circulation of the bath over cation-exchange resins. The basin is composed of a polymer of vinyl chloride, its useful capacity being 1000 liters (B=l000 1.). The cathode is composed of aluminum threads with a surface equal to 40 dm. (anodic surface in dm. =0.04 B=0.04 1000). The ano dic parts to be treated in each operation are aluminum plates of 99.5%

aluminum with a surface of 20 m The'density of an odic current varies from the startto the end of each operation between 0.15 and 0.27 a./dm. Before the first operation, a liter of hydrogen peroxide of 30 volumes concentration is added to the bath.

During 100 days corresponding to 600 operations, the ratio of the concentration of dissolved hexavalent chromium to the trivalent chromium, in atom grams, remains constant from the start of operation, between 125 and 130. The concentration in dissolved aluminum salts remains constant at the value of approximately 1 gram per liter. The anodized surfaces obtained from the first to the 600th operation, have the same satisfying appearance and the same advantageous characteristics as previously described. Since, after these four and a half months of working, the bath has not shown any indication of being used up, as shown by the analyses, the applicant considered the test to be sufficiently conclusive, and considered it useless to continue it longer. One can, however, consider it as certain that its life would exceed 500 days of working, which corresponds to more than 60 m? treated per liter of bath, and consequently to a consumption of the bath below 3.4 grams of chromic acid per m? of treated surface.

Example 2 relates to a known process of anodization in a chromic bath under a constant voltage of 40 volts at 40 C., each operation lasting 40 minutes. The cathode is constituted by bars and plates of pure iron. The surface treated per liter of bath, which is 0.22 dm. cannot be increased without increasing, at the same time, the proportion of trivalent chromium produced per unit time in the bath.

Example 3 relates to a known process of anodization in a sulphuric bath under a constant voltage of 15 v. at 20 C., each operation lasting 40 minutes.

Example 4 relates to the invention. It is not described in the above table. It constitutes a modification of Example 1 and differs therefrom in that it does not have recourse to any addition of hydrogen peroxide nor to purging (fixing) of the aluminum salts of the bath by a continuous passage thereof over cation-exchange resin.

In this case, the ratio Cr6+/Cr3+ of the bath reaches its equilibrium value between 120 and 130 from the very first operation, but it becomes necessary, as seen above, to extend for a longer time the duration of this first operation (duration of the first operation=150 minutes). The following operations last the same time as in Example 1, namely 90 minutes. The ratio Cr6+/Cr3+ is maintained practically unchanged at its value given above until the end of the experiment, of 300 operations corresponding to 6 In. of treated surface per liter of bath. Beyond this limit, the bath ceases to be usable in that it is too highly enriched in aluminum salts. This bath may, however, be employed again in practice as if it were a new bath, provided that the aluminum salts are eliminated by passage over a cation-exchange resin. However, if it is impossible to use such a purging device, the bath must then be renewed. The consumption owing to the renewal of the bath corresponds in this case to approximately 34 grams of chromic acid per m? of treated surface. Even in this unfavorable case, this consumption remains much less to that involved with the earlier practice. This consumption must be compared with the corresponding consumption of an installation provided with a purging device for aluminum salts (Example 1), which is below 3.4 per m The table given above, corresponding to Examples 1 to 3, illustrates principally the exceptional life of the bath, still usable after having anodized a surface of 60 m? per liter of bath (Example 1). The comparison of this figure with the corresponding figure of Examples 2 and 3 shows results of an entirely different order of magnitude, and indicates a surprising technical effect and a marked industrial progress. This progress is translated in the penultimate horizontal column of the table by consumption of starting materials which are low by comparison with the state of the present art.

It is obvious that any process using means equivalent to these described above for the new process falls within the scope of the invention.

That which is claimed is:

1. A process for the surface protection of aluminum and of its alloys by anodic oxidation in an aqueous bath of chromic acid under a constant voltage with a direct current, characterized by the following means employed in combination:

the chromic acid concentration is about 15 to 19% by weight; the bath temperature is below 30 C.; the electrolysis voltage is below 25 volts;

the anodic surface is between substantially 1.7 B dm. and 2.3 B dm. B representing volume of the bath expressed in liters;

the cathodic surface is below 0.06XB dm. whereby the ratio of the number of atom grams of chromium dissolved in the bath in the hexavalent state to the number of atom grams dissolved in the trivalent state remains constant with a value greater than and which may exceed 120.

2. A process according to claim 1, characterized in that the electrolysis is carried out without any agitation.

3. A process according to claim 2, characterized in that the electrolysis voltage is between substantially 21 and 23 volts.

4. A process according to claim 2, characterized in that the cathodic surface is between 0.03 B dm. and 0.05 B dm.

5. A process according to claim 1, characterized in that the bath is subjected to a purging over cation-exchange resin material, which fixes the aluminum salts.

6. A process according to claim 2, characterized in that, prior to the use of the bath, the bath is provided with a proportion of hydrogen peroxide, equal to 0.01 to 0.2% by weight.

7. A process according to claim 2, characterized in that the electrical connection between each object to be anodized and the anode of the cell is realized by laying down the said object on an objects-holder electrically joined with the anode, the electrical connection being by weight of the object.

References Cited UNITED STATES PATENTS 1,771,910 7/1930 Bengough et a1 204-58 X 2,066,327 1/1937 Buzzard 204-58 2,085,002 6/ 1937 Buzzard 204-58 2,788,317 4/1957 Sonnino 204-58 X JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner, 

1. A PROCESS FOR THE SURFACE PROTECTION OF ALUMINUM AND OF ITS ALLOYS BY ANODIC OXIDATION IN AN AQUEOUS BATH OF CHROMIC ACID UNDER A CONSTANT VOLTAGE WITH A DIRECT CURRENT, CHARACTERIZED BY THE FOLLOWING MEANS EMPLOYED IN COMBINATION: THE CHROMIC ACID CONCENTRATION IS ABOUT 15 TO 19% BY WEIGHT; THE BATH TEMPERATURE IS BELOW 30*C.; THE ELECTROLYSIS VOLTAGE IS BELOW 25 VOLTS; THE ANODIC SURFACE IS BETWEEN SUBSTANTIALLY "1.7XB" DM.2 AND "2.3XB" DM.2, B REPRESENTING VOLUME OF THE BATH EXPRESSED IN LITERS; THE CATHODIC SURFACE IS BELOW "0.06XB" DM.2; WHEREBY THE RATIO OF THE NUMBER OF ATOM GRAMS OF CHROMIUM DISSOLVED IN THE BATH IN THE HEXAVALENT STATE TO THE NUMBER OF ATOM GRAMS DISSOLVED IN THE TRIVALENT STATE REMAINS CONSTANT WITH A VALUE GREATER THAN 70 AND WHICH MAY EXCEED
 120. 